Cytokinesis, the final event of the mitotic cycle, involves the formation and constriction of a contractile ring to produce two daughter cells. Despite its importance, little is known about many aspects of this process. Outstanding issues include the composition and structure of the contractile-ring, the signals that control the position of contractile ring formation, the mechanisms involved in recruiting components to the contractile ring, the role of membrane addition during constriction of the contractile-ring, and the regulatory mechanisms that coordinate both mitosis and cytokinesis. The metaphase and cellularization furrows that form during the cortical divisions of early Drosophila embryogenesis are an excellent system to address many of these issues. The furrows that form during these divisions are structurally and functionally equivalent to contractile-rings in conventionally dividing cells. This system is amenable to molecular, genetic, biochemical, and cellular approaches. Of the organisms and cell types in which cytokinesis is being studied, the Drosophila embryo offers one of the most extensive collections of mutations producing specific and distinct disruptions in furrow formation. Much in the way the yeast cdc mutations were used to order cycle cell events, a major goal of this proposal is to use this collection of mutants to establish the dependency relationships and order of events during cytokinesis. Another major focus of this proposal is to understand the in vivo function of two genes, nuclear-fallout (nuf) and grapes (grp). Our lab identified these genes in screens for mutants with metaphase furrow defects. Nuf encodes a centrosome-associated protein that is required for recruiting a distinct set of components to the contractile-ring. Among the experiments described to understand the mechanism of Nuf action, are genetic and biochemical screens to identify components that interact with Nuf. Grp is a serine/threonine kinase and is a homologue of the conserved cell cycle checkpoint Chk1. In addition to a role in timing entry into mitosis, we find that Grp is also specifically required for recruiting Myosin to the contractile ring. Among the experiments proposed to understand the basis of this unsuspected cytoplasmic role of the Grp/Chk1 kinase are genetic and biochemical screens to identify interacting proteins.